1. Field of the Invention
The present invention relates to a fluorescent lamp inverter apparatus.
2. Related Background Art
Conventionally, as a fluorescent lamp inverter apparatus of the type which is provided in a copy machine, a printer or the like to light a fluorescent lamp for illuminating an original and to perform light modulation for such the fluorescent lamp is known. FIG. 4 is a circuit block diagram showing the structure of the conventional fluorescent lamp inverter apparatus. In this fluorescent lamp inverter apparatus, a choking coil L2a is provided to restrict a current flowing in a fluorescent lamp FL1a. Further, the number of turns of a secondary winding n2a of an inverter transformer T1a is set such that a secondary-side output voltage becomes larger than a lighting start voltage Vth of the fluorescent lamp FL1a. Furthermore, when switching elements SW1a and SW2a connected to primary windings n11a and n12a of the inverter transformer T1a are driven in a push-pull mode, the output voltage of a rectangular wave is generated at the secondary winding n2a.
When a peak-to-peak (P--P) value of this rectangular-wave output voltage is larger than the lighting start voltage Vth of the fluorescent lamp FL1a, the fluorescent lamp FL1a lights. Because of a characteristic of the fluorescent lamp FL1a, its impedance .vertline.Z.vertline. before the lighting has a significantly high value but the impedance .vertline.Z.vertline. after the lighting has a relatively small value. Therefore, a discharge current (tube current) after the lighting has a value which is determined by the P--P value and a frequency of the rectangular-wave output voltage and impedance of the choking coil L2a.
A diode bridge DB1a and a switching element SW3a for the light modulation are connected to both ends of the fluorescent lamp FL1a of the secondary winding n2a. A light modulating circuit 18a controls duty ratio of on/off of the switching element SW3a by using a driving signal to perform the light modulation for the fluorescent lamp FL1a. Further, a preheating circuit 13a controls a preheating voltage applied to a filament of the fluorescent lamp FL1a.
However, the above-described conventional fluorescent lamp inverter apparatus has a following problem. That is, take notice of the choking coil L2a. When the fluorescent lamp FL1a is in the lighting state, a voltage substantially equal to the lighting start voltage Vth of the fluorescent lamp FL1a is being applied to the choking coil L2a. Thus, as an inductance value of the choking coil L2a, the sufficiently large inductance value is required to set the tube current of the fluorescent lamp FL1a having a desired value.
Generally, the lighting start voltage of the fluorescent lamp used in the copy machine or the like for illuminating the original is about several hundreds volts (V) (P--P value), the tube current is about several hundreds amperes (A), and an oscillation frequency is about 20 KHz. Therefore, it is required the choking coil of which inductance is about 20 mH (milli-henry), current value is about several hundreds milli-amperes, and winding withstanding voltage is about several hundreds volts. For this reason, the fluorescent lamp inverter apparatus which contains the choking coil satisfying such a specification becomes extremely large in size and high in cost.
In consideration of this problem, the applicant of the present application previously proposed a fluorescent lamp inverter apparatus having two choking coils. FIG. 5 is a circuit block diagram showing the structure of the fluorescent lamp inverter apparatus having the two choking coils. One of the two-divided choking coils is a high-withstanding-voltage and low-current choking coil L3b which is provided on a secondary winding to start lighting of a fluorescent lamp FL1b, and the other is a low-withstanding-voltage and high-current choking coil L2b which is provided on the secondary winding to maintain a tube current.
In order to shift a state of this fluorescent lamp inverter apparatus from a non-lighting state to a lighting state, while a switching element SW3b is in an off state, primary windings n11b and n12b of an inverter transformer T1b are driven by a switching elements SW1b and SW2b in a push-pull mode. At that time, a rectangular-wave output voltage is generated on secondary windings n2b and n3b of the inverter transformer T1b according to their winding ratio.
Since the switching element SW3b is in the off state, a loop (secondary winding n2b choking coil L2b fluorescent lamp FL1b secondary winding n3b choking coil L3b secondary winding n2b) is formed. The output voltage generated on the secondary windings (n2+n3) is applied to the fluorescent lamp FL1b. When the applied voltage is equal to or higher than a lighting start voltage Vth, the tube current restricted by the choking coils (L2+L3) flows in the fluorescent lamp FL1b.
When the switching element SW3b is turned on in the state that the tube current is flowing, two loops respectively containing the secondary windings n2b and n3b are formed. In the one loop (secondary winding n3b.fwdarw.choking coil L3b.fwdarw.switching element SW3b.fwdarw.secondary winding n3b), a reactive current flows. In the other loop (secondary winding n2b.fwdarw.choking coil L2b.fwdarw.fluorescent lamp FL1b.fwdarw.switching element SW3b.fwdarw.secondary winding n2b), the tube current flows.
Since a tube voltage of the fluorescent lamp FL1b is sufficiently lower than the lighting start voltage Vth during the lighting of the fluorescent lamp, the number of turns of the secondary winding n2b is set such that the voltage generated on the secondary winding n2b has a value sufficiently lower than the lighting start voltage Vth and inductance of the choking coil L2b has a sufficiently low value, whereby the desired tube current can be obtained. Further, since only the voltage which is generated on the secondary winding n2b and sufficiently lower than the lighting start voltage Vth is applied to the choking coil L2b, the withstanding voltage can be designed to be low.
On the other hand, the number of turns of the secondary winding n3b is set such that such the number is sufficiently larger than the number of turns of the secondary winding n2b and the voltage generated on the secondary windings (n2+n3) has the value higher than the lighting start voltage Vth, whereby the lighting of the fluorescent lamp can be assured. Therefore, by setting the choking coil L3b having the sufficiently large inductance, even if the tube current at the lighting start time merely has the value sufficiently smaller than that of the desired tube current, the fluorescent lamp is lighted.
That is, such the fluorescent lamp inverter apparatus has a system in which the fluorescent lamp is initially lighted darkly and then lighted brightly by turning on the switching element SW3b.
The reactive current in the loop (secondary winding n3b.fwdarw.choking coil L3b.fwdarw.switching element SW3b .fwdarw.secondary winding n3b) on the side of the secondary winding n3b can be ignored as a whole, because the choking coil L3b is set to have the large inductance. In this case, before the fluorescent lamp is lighted, it is necessary to sufficiently heat its filament by a preheating circuit 13b.
In such the conventional fluorescent lamp inverter apparatus using the two choking coils, since the choking coil L3b which is used in the loop to assure the lighting of the fluorescent lamp is the high-withstanding-voltage, high-inductance and low-current coil, its size became inevitably large.